1. Field of the invention
This invention relates to a semiconductor laser device which has an extremely low threshold current level and can be readily produced by molecular beam epitaxy or metal organic-chemical vapor deposition.
2. Description of the prior art
In recent years, single thin crystal film growth techniques such as molecular beam epitaxy (MBE) and metal organic-chemical vapor deposition (MOCVD) have been rapidly advanced. By these growth techniques, it is possible to obtain epitaxial growth layers of extreme thinness, on the order of 10 .ANG.. Due to the progress in these crystal growth techniques, it is possible to make laser devices based on device structures having very thin layers, which could not be easily manufactured by conventional liquid phase epitaxy. A typical example of these laser devices is the quantum well (QW) laser which has an extremely low threshold current level and in which the active layer has a thickness of 100 .ANG. or less, resulting in the formation of quantum levels therein. Particularly, a single quantum well laser device of a GRIN-SCH (Graded Index-Separate Confinement Heterostructure) type has a very low threshold current level, for example, 350 A/cm.sup.2 (cavity length: 250 .mu.m) and 200 A/cm.sup.2 (cavity length: 500 .mu.m). This has been reported by W. T. Tsang, Applied Physics Letters, vol. 40, 1981, p. 217.
Further, in order to operate a semiconductor laser device at a low current level, the injected current must be confined in the laser oscillation region. Known examples of semiconductor laser devices showing such a current confining function are a VSIS (V-channeled Substrate Inner Stripe) laser device and a BH (Buried Heterostructure) laser device.
FIG. 4 shows a cross sectional view of a conventional VSIS laser device. Referring to FIG. 4, the structure of the VSIS laser device and the method of producing the device will be described.
On the surface of a semiconductor substrate 1, a current blocking layer 6 is formed. A V-channel which reaches to the substrate 1 is formed in the current blocking layer 6 by an etching technique to form a narrow current path. Then, on the current blocking layer 6 including the V-channel, a cladding layer 2, an active layer 3, a cladding layer 4 and a cap layer 5 are successively formed by a liquid phase epitaxy (LPE), which shows very excellent characteristics of covering a surface having a step(s). As a result of forming the V-channel, the current path is made narrow so that the unstable oscillation mode is eliminated and the device oscillates in the fundamental traverse mode. The VSIS laser device in which the flat active layer 3 is sandwiched by the cladding layers 2 and 4 to form a double heterojunction has a threshold current of 40 mA or less when the width w of the V-channel is 4 .mu.m.
Various kinds of index-guided semiconductor laser devices of this type have been widely developed using an LPE technique. Such a laser device is produced by forming crystalline layers on a substrate having a channel or mesa. Therefore, it is usually difficult to produce such a laser device by MBE or MOCVD. A laser device of this kind has a defect that the active region having a quantum well structure cannot be formed, thereby inhibiting the operation of the laser device at a low current level.
FIG. 5 shows a cross sectional view of a BH laser device. Referring to FIG. 5, the structure of the BH laser device and the method of producing the device will be described.
On the semiconductor substrate 1 of a first conductivity type, a lower cladding layer 2 of the first conductivity type, an active layer 3 and a upper cladding layer 4 of a second conductivity type are successively formed by MBE or MOCVD. Then, the wafer is etched to form a mesa-shaped double heterojunction laser oscillation region including the cladding layers 2 and 4 and active layer 3. A first burying layer 8 of a second conductivity type and a second burying layer of a first conductivity type are successively formed on the surface of the substrate 1 to bury the oscillation region. Then, a cap layer 5 is formed. The refractive indexes of the burying layers 8 and 9 are selected to be smaller than the effective refractive index of the laser oscillation region so that the laser light can be sufficiently confined in the mesa-shaped oscillation region.
As the burying layers 8 and 9 are reversely biased during the operation of the laser device, the injected current cannot pass the burying layers 8 and 9 and are confined in the laser oscillation region, resulting in the formation of a narrow current path. In this index-guided BH laser device, the current and light are confined within the oscillation region by the burying layers 8 and 9, thereby achieving a laser device with a low operating current. A semiconductor laser device of this type having a stripe width (a width of a mesa region) w of 2 .mu.m or less can achieve a low threshold current level of 10 mA or less.
The threshold current level of the BH laser device may be lowered further by replacing the active region with one having a quantum well structure and decreasing the width w. However, unless, the height of the interface (p-n junction) of the first and second burying layers 8 and 9 coincides with that of the interface between the active layer 3 and the second cladding layer 4, as shown in FIG. 4, it is very difficult to lower the threshold current level to about 1 mA because of the following reason. When the height of the interface between the first and second burying layers 8 and 9 fails to coincide with that of the interface between the active layer 3 and the second cladding layer 4, paths of ineffective current which do not contribute to the laser oscillation are formed as shown by the arrow s in FIGS. 6A and 6B. This ineffective current, which amounts generally to about 1 to 5 mA, is larger than the current necessary for laser oscillation, causing the main factor of inhibiting the production of a semiconductor laser of a low threshold current level.
Even if the height of the interface between the first and second burying layers 8 and 9 coincides with that of the interface between the active layer 3 and the second layer 4, it is impossible to suppress the amount of the ineffective current flowing from the active layer 3 to the burying layer 8 or 9 to an extremely low level of about 1 mA or less.
As seen from the above, in conventional VSIS semiconductor laser devices, it is very difficult to form an active area of a quantum well structure by MBE or MOCVD. In conventional BH semiconductor laser devices, it is possible to form an active area of a quantum well structure by MBE or MOCVD, but it is very difficult to coincide the height of the interface of the active area with that of the p-n junction formed by the burying layers. Even if the heights are coincided with each other, it is very difficult to prevent the leakage of the injected current from the active area to one of the burying layers. In any case, it is difficult to produce a semiconductor laser device of a low threshold current level by a conventional technique.